I think that the ability to take bending away was given to Korra as an extension of Aang. Aang was taught how to energybend by the Lion Turtles, and he used this technique through Korra. However with Korra's severed connection to her past lives, I don't think she would have been able to complete energybending without first visiting the Lion Turtles.
Also, platinum is the weakness to metalbending; it is the most refined of metals, to the point where there's hardly any earth left in it to bend. This was seen back in Season One, in which the mecha tanks were made out of platinum so they could stand against metalbenders.
With the Red Lotus being a rather insane anarchist cult, as well as probably only being formed/reformed in the weeks/few months after Zaheer's team's prison escape, it's pretty likely that they would only be able to find a few minorly skilled volunteers.
That being said, what I personally do not get is why Aang somehow never learned metalbending from Toph. I assume it's just because he got busy with the New Republic work with Zuko, but Toph was directly his earthbending master. You'd think he'd at least try to work with such a valuable skill.
Also, I was extremely disappointed that Zuko did not at least once show that he had mastered lightning/blue fire, or show off the "Dragon's Breath" move.